Method for heat treating steel strip



J. T. MAYHEW EAT TREATING STEEL STRIP METHOD FOR H Filed April 2, 1964 Q N \E 5528 I l Qz QT kzmzmmn ws mmsEmmmzwp 05.2w: o o o o m m m m JOHN T. MAYHEW A TTORN E Y5.

United States Patent 3,297,499 METHOD FOR HEAT TREATING STEEL STPJP John T. Mayhew, Toronto, Ohio, assignor to National Steel Corporation, a corporation of Delaware Filed Apr. 2, 1964, Ser. No. 357,861 5 Claims. (Cl. 148-156) This application is a continuation in part of patent application Serial No. 163,702, filed January 2, 1962, now abandoned.

The invention is concerned with methods and apparatus for producing and heat treating steel strip. Of special concern is relief of internal stress, cooling strains, coating strains and embrittlement, strain aging, and the quench aging characteristics in steel strip which develop or manifest themselves after annealing of the strip.

Steel strip which is to be used for deep drawing purposes is ordinarily annealed to relieve the strains developed in hot and cold rolling and to soften the steel and improve its ductility. Annealing may be performed continuously as part of a continuous strip hot-dip operation or the strip may be batch-annealed before being introduced to a coating line. In either event the annealed strip is ordinarily introduced into a hot-dip coating pot in a heated condition in order to improve coating adherence and for other purposes well known in the art. Defects produced in steel strip by such heating, and by other coating practices, are relieved by the invention.

In describing the invention reference will be had to the accompanying drawing which schematically illustrates a continuous strip hot-dip coating line embodying the invention. A continuous strip hot-dip galvanizing operation will be described specifically although the invention is applicable to other hot-dip metal coating operations. In galvanizing, steel strip 1 is introduced into coating pot 2 through chute 4. The temperature of the strip may be measured and controlled by temperature control unit 6 which may control strip heating within chute 4 or may control heating earlier in the line. Generally strip 1 is heated to a temperature such that it adds to molten galvanizing bath 8 in coating pot 2. Molten bath 8 ordinarily comprises zinc and a small percentage of other metals such as aluminum, load, tin, etc. and is maintained at a temperature around 800 to 850 F. Strip 1 passes around sinker roll 10 and upwardly through coating control means 12 located at the surface of the molten galvanizing spelter and emerges from the molten bath 8 as coated strip 14. The temperature of coated strip 14 as it emerges from molten bath 3 is approximately equal to the temperature of the molten galvanizing spelter. Coated strip 14 travels upwardly through cooling tower 16 where it is cooled and its coating solidified before contact with top roll 18. Cooling tower 16 may employ forced air cooling or merely ambient air circulation depending upon temperature of the strip, gage of the strip, ambient conditions, and other factors. The temperature of coated strip 14 will ordinarily be around 600 F. when it reaches top roll 18. Then strip 14 travels downwardly to return roll 29 and may be washed, leveled, and stamped thereafter. Conven-tionaily strip 14 will reach a temperature around 150 F, before coiling.

The invention deals chiefly with defects developed in the steel after annealing and before final coiling of the coated product or with problems that manifest themselves after production, such as age hardening of the steel. The defects stem from handling, heating, cooling, and other steps in hot-dip coating practice and are known variously as cooling strains, internal stress, galvanizing strain, galvanizing embrittlement, strain aging and the like. Such defects are manifested. predominantly, as increased hardness in the strip and decreased ductility of the strip.

defined in the art.

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Hardness and ductility are of prime importance in deep drawing steel strip and are of chief concern in the invention although other steel physicals such as yield point and percent elongation are affected by the invention. The definitions of these physical properties of steel and the part they play in deep drawing steel strip are well known in the art so that no detailed discussion is necessary to an understanding of the invention. In general however, relatively soft steel with high ductility, low yield point, and a high percentage elongation is desired in deep drawing steels.

In the past the coating itself made hot-dip metal coated steel strip unsuitable for deep drawing operations. Hotdip metal coated steel could not take deep drawing without cracking or flaking of the coating. However, with advances in coating practice, many coated fiat rolled steel products, such as galvanized strip, have been used in creasingly in deep drawing applications. As an example of the latter, automobile bodies are ordinarily deep drawn and there is an increasing demand for galvanized steel rocker panels, and the like. Considering 18 gage strip for such purposes, requests for steel having a hardness of Rockwell 50 (B scale) and a ductility of Olsen 420 are common. (Rockwell and Olsen numbers used herein are from the standard, well known Rockwell Hardness Tests and Olsen Cup Tests practiced in the art). Rockwell 50 (B scale) and Olsen 420 for 18 gage strip may be end limits and often softer, more ductile steel is desired. However, conventional 18 gage strip coming from continuous galvanizing lines can easily have a Rockwell (B scale) number of or higher and an Olsen number of 407 or lower. It is the primary objective of the invention to provide methods and means for heat treating hot-dip :rnetal coated steel strip to decrease its hardness and increase its ductility without damage to the metal coating. Lower yield point and higher percent elongation for the steel strip follow from the treatment of the invention and the problem of age hardening of the steel, commonly referred to as quench aging, is, for practical purposes, eliminated by the invention.

The deep drawing steel strip concerned, which may be killed, semikilled, capped or rimmed steel, is termed very low carbon steel and may have a carbon content ranging from about 0.04 to about 0.15% although 0.05 to 0.06% carbon content is most common. During coating operations these steels develop the defects previously noted. While these defects are well known in the art, causes or explanations for them are not well known or Some attribute them to the handling of the strip after annealing; some attribute them to the cooling practice after coating, and some attribute them to the precipitation of carbon from a supersaturated ironiron carbide solution existing in the steel after annealing. There is general agreement however, that where manifested as an increase in hardness, this increase in hardness is accelerated with strain, such as that involved in handling during coating operations and is accelerated with the heating and cooling practice of coating operations. Also it has been widely observed and noted that the strip will harden with age, usually by six or more Rockwell (B scale) numbers, during the first 30 to 45 days after production. Much etfort is being expended in post annealing and post coating operations in order to avoid the above defects but with little success. One method, currently experimental, involves long gradual cooling of strip after coating. Some improvement in hardness has been noted but no relief of age hardening occurs. Also, most continuous strip lines are not equipped to provide the long duration temperature gradient required. The invention does away with such requirements and permits handling, heating, and cooling practices best suited to the coating operation without concern for development of the previously noted defects. Also the apparatus can be readily installed and the methods readily applied on conventional continuous hot-dip metal coating lines without significant overhaul.

In accordance with the invention the strip is cooled after coating, either by the time it reaches the top roll 18, the return roll 20, or thereafter, to a temperature below about 400 F., is reheated and then heat soaked at a temperature above about 600 F.

Certain temperature ranges of operation for the steps enumerated are permissible Within the teachings of the invention to obtain the desired results. Good results have been experienced in cooling the strip after galvanizing to as low as ambient temperature. On many lines, the strip will ordinarily be cooled to about 150 F. This temperature, before reheating, is satisfactory, in fact, temperatures below 200 F. are especially effective. As an upper limit on the cooling temperature, the strip should be cooled to below about 500 F.

On reheating, the range of temperatures extends from above about 600 F. up to the melting temperature of the coating, the latter temperature playing an important role in the type of heating apparatus employed which will be discussed in more detail in subsequent paragraphs. The preferred temperature range for reheating is between about 600 F. and about 800 F. An optimum reheating temperature for most deep drawing steels is 700 F.

For heat soaking, the range of temperatures should not include a temperature close to the melting temperature of the coating otherwise the heat soaking, while beneficial to the steel, may damage the coating. To heat soak by coiling, the coiling temperature range should extend from above about 500 F. to a temperature somewhat below the melting temperature of the coating. The range of coiling temperatures for galvanized strip can extend from above about 500 F. to about 750 F.

No categorical explanation can be offered why the cooling, reheating, and heat soaking combine to produce the desired results. It is believed that the reason it is necessary to cool below about 500 F. is in order to provide an opportunity for deficiencies in the steel to become somewhat set. Otherwise they remain latent in the steel and reheating Without preliminary cooling is not effective. It may be that some precipitation hardening is necessary before the reheating can be effective. Also, it may :be thatthe heating at first accelerates hardening and carbon precipitation and then coalesces the carbon into particles too large to act as effective hardening agents. No significant change in the size of the iron crystals is believed to take place at the reheating temperatures but it is believed there is relief of strain Within the iron crystals. The heating need not be prolonged to be effective as the strip softens rapidly at certain temperatures; however, the strip should be coiled above about 500 F. to permit heat soaking. While heat soaking by coiling above about 500 F. is best suited to the environment of continuous strip coating lines other methods of heat soaking may be employed. The heat soaking need not be prolonged, since a good portion of the beneficial results are obtained in the first minute of soaking. For most purposes a heat soaking of one to ten minutes will suffice.

The softening effect on 18 gage steel strip treated in accordance with the invention is shown in the following examples:

Olsen Cup Test,

Edge Center Edge Before 431 427 423 After 440 435 438 The teachings of the invention are not to be limited to any particular gage strip by these examples but rather should be considered applicable to any of the commercially required gages of steel strip.

Referring to the drawing, after passing return roll 20. coated strip 14 is fed through an electrical heating unit 22 and onto rewind coil 24. The apparatus employed in carrying out the invention is important in order to avoid possible damage to strip coating. The strip is heated from within in order to avoid scorching, discoloration or other marring of the coating and to avoid damage to coating adherence. Induction heating is considered most satisfactory for this purpose. Induction heating concentrates heat in the base metal whereas with radiant, conduction, or convection heating it is necessary to force the heat through the coating. It is believed that, with electromagnetic induction, the energy transfer is concentrated in the base iron, which is ferromagnetic, rather than the coating metal, such as zinc, which is diamagnetic. Therefore the steel strip can be brought quickly up to the desired temperature without melting the coating or increasing the alloying 'between the coating and the strip. The frequency of the electromagnetic induction should be chosen to permit the greatest heat producing penetration of the base metal; the power and number of units required will depend on the gage of the strip and the speed of the line; these factors are within the skill of an electrical engineer skilled in the art of induction heating. Heating need not be limited to induction heating, for example resistance heating can provide the necessary heating from within without damage to the coating.

It is understood that the invention is not to be limited by the specific description included hereabove but may be practiced otherwise within the scope of the appended claims.

What is claimed is:

1. Continuous-strip method for heat treating steel strip after coating in a hot-dip galvanizing bath to relieve effects such as cooling strains, galvanizing strains, galvanizing embrittlement, strain aging or quench aging characteristics comprising removing galvanized strip from a hot-dip galvanizing bath,

cooling the galvanized strip to a temperature in a temperature range having a lower limit about atmospheric temperature and an upper limit about 500 F. heating the cooled strip to a temperature within a temperature range having a lower limit about 600 F. and an upper limit about 800 F. but below the melting temperature of the galvanize coating, and then coiling the strip While at a temperature within a temperature range having a lower limit about 500 F. and an upper limit about 750 F. but below the melting temperature of the galvanize coating.

2. Continuous-strip method for heat treating teel strip after coating in a hot-dip galvanizing bath to relieve effects such as cooling strains, galvanizing strains, galvanizing embrittlement, strain aging or quench aging characteristics comprising removing galvanized strip from a hot-dip galvanizing bath, cooling the galvanized strip to a temperature in a temperature range having a lower limit about atmospheric temperature and an upper limit about 500 F.

inductively heating the cooled strip to a temperature within a temperature range having a lower limit about 600 F. and an upper limit about 800 F. but below the melting temperature of the galvanize coating, and then coiling the strip while at a temperature within a temperature range having a lower limit about 500 F. and an upper limit about 750 F. but below the melting temperature of the galvanize coating. 3. Method for relieving eflects such as cooling strains, strain aging or quench aging characteristics of steel strip coated with metallic material having a melting point above 600 -F. comprising removing coated steel Strip from a coating operation at a temperature above 600 F.,

cooling the strip to a temperature within a temperature range having a lower limit about atmospheric temperature and an upper limit about 500 F.,

heating the cooled strip to a temperature within a temperature range having a lower limit about 600 F. and an upper limit approaching but less than the melting temperature of the coating, and then heat soaking the heated strip at a temperature within a temperature range having a lower limit about 500 F. and an upper limit approaching but substantially less than the melting temperature of the coating for a period greater than one minute.

4. Method for heat treating steel strip to eliminate 11ndesirable mechanical properties stemming from handling, heating, and cooling of continuous strip during processing comprising continuously annealing steel strip,

cooling the continuously annealed strip to a tempera ture below 500 F. during processing of the strip such that undesirable mechanical properties are developed,

subjecting the cooled strip to a continuous-strip heating operation in which the strip is heated to a temperature within a temperature range having a lower limit about 600 F. and an upper limit about 800 F., and then heat soaking the strip by coiling the heated strip directly after the heating step with successive convolutions of the strip in contact with one another, the heat soaking temperature being within a temperature range having a lower limit about 500 F. and an upper 'limit about 800 F., maintaining the strip under conditions such that the coiled strip is within the heat soaking temperature range for a period of time to eliminate the undesirable properties.

5. The method of claim 4 in which the continuously annealed strip is cooled to atmospheric temperature prior to the heating step.

References Cited by the Examiner UNITED STATES PATENTS 7/1957 Eckel 148134 X 4/1962 Beattie et al 148-127 OTHER REFERENCES Metals Handbook, American Society of Metals, 1948 edition relied on, pages 352-357. 

4. METHOD FOR HEAT TREATING STEEL STRIP TO ELIMINATE UNDESIRABLE MECHANICAL PROPERTIES STEMMING FROM HANDING, HEATING, AND COOLING OF CONTINUOUS STRIP DURING PROCESSING COMPRISING CONTINUOUSLY ANNEALING STEEL STRIP, COOLING THE CONTINUOUSLY ANNEALED STRIP TO A TEMPERATURE BELOW 500*F. DURING PROCESSING OF THE STRIP SUCH THAT UNDESIRABLE MECHANICAL PROPERTIES ARE DEVELOPED, SUBJECTING THE COOLED STRIP TO A CONTINUOUS-STRIP HEATING OPERATION IN WHICH THE STRIP IS HEATED TO A TEMPERATURE WITHIN A TEMPERATURE RANGE HAVING A LOWER LIMIT ABOUT 600*F. AND AN UPPER LIMIT ABOUT 800*F., AND THEN HEAT SOAKING THE STRIP BY COILING THE HEATED STRIP DIRECTLY AFTER THE HEATING STEP WITH SUCESSIVE CONVOLUTIONS OF THE STRIP IN CONTACT WITH ONE ANOTHER, THE HEAT SOAKING TEMPERATURE BEING WITHIN A TEMPERATURE RANGE HAVING A LOWER LIMIT ABOUT 500*F. AND AN UPPER LIMIT ABOUT 800*F., MAINTAINING THE STRIP UNDER CONDITIONS SUCH THAT THE COILED STRIP IS WITHIN THE HEAT SOAKING TEMPERATURE RANGE FOR A PERIOD OF TIME TO ELIMINATE THE UNDESIRABLE PROPERTIES. 